PCL82 Valve Heater Voltage

[bobhowe]

Hi Would it be ok to power a PCL82 valve heater at 15 Volts ? kind regards Bob

[Mike Watterson]

Yes, the normal limits are +/-10% though -10% greatly increases life and +10% decreases it. The 15V is a reduction of 6.25 %.
But what tolerance is the 15V? It should be greater than 14.4V which is -4% on 15 V

Everything varies and has a range.

However likely a PCL82 would work of a 12V lead acid, which is about 10.8V (below that damages the battery to a smidge above 14V fully charged, which is why a bench PSU for 12V battery gear is a nominal 13.8 V.

Reducing the heater voltage slightly reduces gain and gradually limits the maximum anode current.

Really only excess heater voltage will damage a valve, a somewhat reduced voltage where it still "goes" doesn't harm it.

[Mike Watterson]

Because the PCL82 was designed for TV use the triode section is larger than needed for an AF pre-amp. The ECL82 was developed from it and has the same oversize triode.

An interesting experiment would be triodising the pentode section (connect g2 to anode) and use a 100V to 150V HT and a mains transformer that's for 220-240 or 110-120 and wiring that as push pull. Adjust the series resistor on each cathode for about 1mA to 5mA, adjust the antiphase drives so output level the same on each part. The bias currents cancel, so no transformer saturation. The 6V to 12V (or 2 x 6V windings) secondary drives speaker.

However quite a few "signal" valves can do 500mW, or even 2W push pull. EF80, either section of an ECH81/ECH83 (same valve). But not ECC81, ECC85 etc.

[Mike Watterson]

Here is a simple tape recorder that used the PCL82
https://www.radiomuseum.org/r/amroh_hand...s256h.html

Some taperecorders used the playback audio output pentode as the "ultrasonic" oscillator for the erase head and recording head bias.
bias/erase 37.65 kHz, frequency range up to 10 kHz, combined Amroh record-play-erase head, 1 AC motor.
2 W audio. No doubt the EM34 was for recording level. From 1935 to 1965 a magic eye was cheaper than a sensitive moving coil meter, hence common from 1945 to 1965 as recording level meters in tape-recorders.

[bobhowe]

Hi Mike thanks for advise and that is one nice tape recorder looks a lot easier to work on than some BSR variants kind regards Bob

[Geordie McBoyne]

Hi Bob,
the PCL82 should be OK with 15volts as the heater voltage. The valve was introduced in 1954 for use in TV receivers having 90 degree deflection CRTs.
Information from the 1962 Mullard Maintenance Manual.

Geordie McBoyne.

[Mike Watterson]

The BSR are MUCH easier than the same era Grundig. I've done both. Of course the Grundig had x2 size motor and 3x as many valves and ALC.

Have fun and one hand in pocket! Also the PCL82 isn't the worst but gets pretty hot! About 4.5W just for the heater and glass doesn't radiate heat too well. Not as bad as picking up the soldering iron by the wrong end.

Beginners Wand has two labels:
1. hold this end.
2. point this at object to be enchanted.

We are magicians and the soldering irons are our wands.

[Mike Watterson]

Information from the 1962 Mullard Maintenance Manual.

So 5W on Pentode, 1W on Triode and 4.5W = 10.5W

Looking at the powers and gain, my idea of about 100V HT (or a bit more) and triodizing pentode, suitable resistors on cathode might work as Class B. Then you'd get about 4W audio and higher efficiency and able to use a regular mains transformer as low bias which cancels anyway.
Rule of thumb for class B push pull (or slightly class AB) is that the max power is about x4 of one device.

A 5W single ended Class A will use 10W of HT and need a real audio transformer able for the bias current.

[bobhowe]

Hi David thanks for information it is going to be another project for the Thorn branded PCL82 valve you gave me last year at Ryhope Classic Car Show last year kind regards Bob

[bobhowe]

Hi Mike someone told me a while ago that the PCL82 runs hot it might save me putting the heating on going to start a new thread as I have got a PCB for a single ended mono amplifier kind regards Bob